Investigation of rock fragmentation during rockfalls and rock avalanches via 3-D discrete element analyses

This paper investigates the characteristics of dynamic rock fragmentation and its influence on the postfailure fragment trajectory. A series of numerical simulations by discrete element method (DEM) were performed for a simple rock block and slope geometry, where a particle agglomerate of prismatic shape is released along a sliding plane and subsequently collides onto a flat horizontal plane at a sharp kink point. The rock block is modeled as an assembly of bonded spherical particles with fragmentation arising from bond breakages. Bond strength and stiffness were calibrated against available experimental data. We analyzed how dynamic fragmentation occurs at impact, together with the generated fragment size distributions and consequently their runout for different slope topographies. It emerges that after impact, the vertical momentum of the granular system decreases sharply to nil, while the horizontal momentum increases suddenly and then decreases. The sudden boost of horizontal momentum can effectively facilitate the transport of fragments along the bottom floor. The rock fragmentation intensity is associated with the input energy and increases quickly with the slope angle. Gentle slopes normally lead to long spreading distance and large fragments, while steep slopes lead to high momentum boosts and impact forces, with efficient rock fragmentation and fine deposits. The fragment size decreases, while the fracture stress and fragment number both increase with the impact loading strain rate, supporting the experimental observations. The fragment size distributions can be well fitted by the Weibull's distribution function. Plain Language Summary Fragmentation occurring in rapid rock mass movements due to shearing of highly stressed grain chains and extremely rapid loading could be described as dynamic fragmentation. In this study, we present a series of simulations by discrete element method for a simple block and slope geometry, where a particle agglomerate of prismatic shape is released along a sliding plane and subsequently collides onto a flat horizontal plane at a sharp kink point. The fragmentation is intense in the lower frontal part of the rock block, whereas the upper part suffers little from impact, generating relatively large fragments. The sudden impact causes the major damage of rock mass, whereas subsequent sliding and collision lead to further rock fragmentation. At impact, the vertical velocity decreases, while the horizontal velocity increases sharply, facilitating the transport of fragments along the bottom floor. The slope angle influences rock fragmentation by controlling the momentum distribution in the sliding and colliding processes. Gentle slopes normally lead to long spreading distance and large fragments, while steep slopes lead to high momentum boosts and impact forces, with efficient rock fragmentation and finer deposits. The role of strain rate on the degree of fragmentation is studied, and numerical results support the experimental observations.